Unbalanced forces act on an object when the forces applied to it do not cancel each other out, leading to a net force. In the situations given:
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Two people stand on the same side of a large tire. Both people pull the tire with equal force. (Balanced forces: no net force)
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Two people stand on the same side of a large tire. One person pushes the tire and the other pulls the tire with equal force. (Unbalanced forces, since one is pushing and the other is pulling in opposite directions)
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Two people stand on opposite sides of a large tire. Both people pull the tire with equal force. (Balanced forces: no net force)
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Two people stand on opposite sides of a large tire. Both people push the tire with equal force. (Balanced forces: no net force)
Thus, the situation where unbalanced forces are acting on an object is the second one.